This application is related to copending applications Ser. No. 298,838, P. D. Johnson and S. D. Silverstein and Ser. No. 298,837, S. D. Silverstein, both filed on the same date as the present application and assigned to the same assignee as the present invention.
This invention relates to high-pressure sodium lamps. More specifically, the invention relates to improvement of high-pressure sodium lamp efficacy through the combined effect of increased arc tube diameter and use of improved IR reflective film to maintain arc tube wall temperature in the optimum range.
A high-pressure sodium lamp generally comprises an inner arc tube disposed within an outer protective envelope and which contains the conventional ionizable medium of sodium, mercury, and an inert gas to facilitate start-up. As current is passed through the electrodes located at each end of the arc tubes, the inert gas ionizes and forms an arc between the electrodes. The sodium vaporizes due to the heat of the arc. Optimum operating arc tube wall temperature of such lamps is in the range of 1400.degree. K. to 1500.degree. K. The arc tube diameter of a conventional 400 watt high pressure sodium lamp is approximately 7 millimeters.
An important consideration in the design of high-pressure sodium lamps is the "wall load" parameter, defined as power per unit area. In practical terms the "wall load" is measured by dividing the lamp power input by the area of the interior surface of the arc discharge tube. The importance of the wall loading is due to its significant effect on arc tube wall temperature, which, in turn, is closely related to lamp efficacy (measured in lumens/watt). Hence, the desirability of maintaining a predetermined optimal arc tube wall temperature in a high pressure sodium lamp is quite apparent.
J. F. Waymouth and E. F. Wyner have demonstrated, as described in a paper presented at the annual meeting of the IES (August 1980), that the efficacy of a high-pressure sodium lamp is improved with increased arc tube diameter at constant arc tube wall temperatures. In a conventional high-pressure sodium lamp, a significant fraction of the energy input to the lamp is dissipated as long wavelength IR radiation from the incandescence of the heated alumina (Al.sub.2 O.sub.3) arc tube. However, since the thermal radiation heat transfer is proportional to the area of the arc tube, a larger diameter arc tube (hence, one with a greater area) radiates even more heat. Unless steps are taken to recover the radiated heat, the arc tube wall temperature will fall below the optimum temperature range and more energy must be supplied to the lamp to raise arc tube wall temperature. Moreover, as the sodium concentration is inversely proportional to arc tube wall temperature, the cooler wall temperature will result in a greater reabsorption of the main sodium emission line (NaD) and a lowering of lamp efficacy. The method proposed by Waymouth and Wyner to maintain the arc tube wall temperature of a larger diameter arc tube in the optimum range involves the substitution of yttria (Y.sub.2 O.sub.3) for alumina as the arc tube material (yttria having lower emissivity than alumina, especially in the infrared region of the spectrum).
In accordance with the present invention, the efficacy of a high-pressure sodium lamp having a larger-than-conventional arc tube diameter is increased by deploying a composite IR reflective film made up of indium oxide doped with tin (In.sub.2 O.sub.3 :Sn) or tin oxide doped with fluorine (SnO.sub.2 :F) in combination with dielectric films of titanium oxide (TiO.sub.2) and/or silicon oxide (SiO.sub.2) on the inner surface of the outer lamp envelope. The IR reflective film is substantially transparent to visible radiation but acts to reflect infrared radiation toward the arc tube which would otherwise be lost. A substantial fraction of IR emission from the arc is reflected back into the plasma contained within the arc tube where it is reabsorbed, resulting in the reduction of required input power. TiO.sub.2 and SiO.sub.2 dielectric films in combination with In.sub.2 O.sub.3 :Sn or SnO.sub.2 :F films can decrease the reflectivity of the IR reflective film at the visible wavelengths and increase reflectivity in the near-visible IR wavelengths. The dielectric films also enhance the chemical stability of the IR reflective film at high temperature. In this manner, the arc tube wall temperature is effectively and efficiently maintained in the optimum range.
In the past, IR reflective films have been used with low-pressure sodium lamps as a means to improve efficacy. U.S. Pat. No. 3,400,288 to Groth is illustrative of such lamps. The principle of operation of a low-pressure sodium lamp, however, is unlike that of a high-pressure sodium lamp. Consequently, the mechanism for increasing the efficacy in the low-pressure sodium lamp is also different from that of high-pressure sodium lamp. In the low-pressure sodium lamp, the efficacy increase is a result of the increase in sodium vapor pressure at constant input power due to higher wall temperature. In contrast, the efficacy increase in the high-pressure sodium lamp of the present invention is due to combined effects of: increased arc tube diameter, the use of composite IR reflective film to maintain optimum wall loading, and to reflect part of the nonvisible emission from the plasma back into the plasma. Moreover, increasing the arc tube diameter of a low-pressure lamp is not accompanied by changes in efficacy such as those observed in high-pressure sodium lamps.
U.S. Pat. Nos. 3,931,536 and 3,662,203 to Fohl et al and Kuhl et al, respectively, disclose high-pressure sodium lamps including IR reflective films. The patent to Fohl et al discloses a reflective film made up of alternate layers of titanium oxide (TiO.sub.2) and silicon oxide (Si.sub.2 O). One such reflector consists of thirteen quarter-wave alternate layers of TiO.sub.2 and SiO.sub.2 sandwiched between eighth-wave layers of SiO.sub.2. As may be seen, such a reflector is significantly more complex than the composite IR reflective film employed in the present invention. Kuhl et al proposes additional heating of the arc tube by positioning the outer envelope in very close proximity to the arc tube. The outer envelope, composed of highly refractive quartz, reradiates arc heat back to a quartz arc tube. The method disclosed by Kuhl et al, thus, not only employs a relatively expensive quartz outer envelope, but the reduced surface area of the outer envelope might result in the undesirable overheating of any reflective films deployed on the outer envelope. Moreover, neither Fohl et al nor Kuhl et al show any appreciation of the desirable effect on the efficacy of a high-pressure sodium lamp of increased arc tube diameter and IR reflective In.sub.2 O.sub.3 :Sn or SnO.sub.2 :F film.